1. Field of the Invention
The present invention is related generally to fishways, and in particular to a fishway positioned between a river mouth and a point upstream along the river and in which a salt concentration gradient is created so as to more accurately simulate the transition between salt water and fresh water normally encountered by fish which are migrating to their spawning grounds.
2. Description of the Prior Art
As the world's population expands, mankind has increasingly sought to control more of the sources of fresh water to meet its needs for residential, industrial and agricultural purposes as well as for flood control. The means for taking charge over naturally flowing rivers has been to construct dams, weirs and other manmade obstacles. These barriers have not been helpful for the ecosystem of fish.
The construction of dams, weirs and any man-made obstacles at river mouths presents two challenges for the passage of fish upstream or downstream. The first is the obvious physical barrier preventing the passage of fish. The second challenge is less apparent, but nonetheless daunting. This problem is caused by the sudden change in the salt concentration between salt water and fresh water.
The most common solution to the first problem is to install a fishway that allows the fish to traverse the obstacle. This is built in a low slope which eases their passage. Regulating the water flow also assists the fish.
Up to this point, the second problem has not been solved. In a pristine river environment, brackish water is formed naturally where the river meets the body of water into which it drains. The water at this point is a fluctuating mix of fresh water and salt water and can be affected by the tides. Fish, such as salmon, need to adapt to the difference fresh water and salt water makes on their bodies. Also, brackish water helps to give each river its distinct smell. It is commonly known that some migratory fish memorize the smell of the river in which they were born.
Migratory fish are born upstream and then go to sea until they are adult fish. When they spawn, they return upstream to the general area in which they were born. The introduction of a man-made structure causes the elimination of the brackish water. With the loss of the distinct smell, many fish will not be able to find their way back. If they do make it back to the river, the fish can be deterred from swimming upstream because of the sudden and drastic change in the concentration of salt.
Future fishways need to not only consider the ladder gaps, but to take into consideration in their design the gradation of the salt concentration and the smell that occurs in a river's original features.
Information about the state of the fishway art is provided in the following: U.S. Pat. No. 5,139,364 to Arakeda et al.; U.S. Pat. No. 4,629,361 to Zimmerman; U.S. Pat. No. 4,437,431 to Koch; U.S. Pat. No. 4,260,286 to Buchanan et al.; U.S. Pat. No. 3,962,876 to Phillips; U.S. Pat. No. 3,232,060 to Windle; and Kazuo Ueda, "Physiology of Fish Recurrent Migration," Gakkai Press Center, pp. 172-180, 1987.